A. Field of the Invention
The present invention relates to apparatus for cleaning soiled articles, and more specifically to a high-efficiency immersion cleaning apparatus for removing grease and the like from metal, ceramic or glass articles.
B. Description of the Related Art
A wide variety of manufactured items acquire layers of grease or other contaminants during fabrication, storage or use. Such items include printed circuit boards, electronic components, machined metal parts and assemblies, and components fabricated from plastic, glass and ceramic materials. In order to maintain the usefulness of these items, they must be periodically (or at least initially) rendered free of contaminants.
One traditional method of cleaning manufactured articles is vapor degreasing. This process involves suspension of the soiled article in a dense vapor generated by boiling one of numerous commonly used degreasing solvents. This suspension in solvent vapor may be accompanied by one or more immersion steps.
During suspension in the vapor, the solvent vapor condenses on the relatively cool soiled article, dissolving and rinsing away soluble contaminants along with entrained non-soluble contaminants. The article absorbs the solvent's heat of vaporization until its surface temperature approaches that of the solvent vapor. At this point, equilibrium is established between the wet article surface and the surrounding vapor, and further condensation ceases. The article may then be removed from the vapor, largely free of both soil and excess surface solvent, and ready for further processing. Degreasing apparatus that provide for subsequent immersion steps may also include ultrasonic transducers to promote mechanical and hydraulic solvent action, producing a scrubbing effect that frees recalcitrant or otherwise inaccessible contaminant.
Conventional degreaser designs feature a housing that contains a boiling sump to generate the solvent vapor, and a series of cooling coils located toward the top of the housing that continuously condense the rising vapors. This action both preserves solvent and prevents environmentally hazardous rates of solvent-vapor emission. Before the condensate is returned to the boiling sump (or immersion basin, if the device is so equipped), it usually passes through a separation device that removes water acquired by the vapor and cooling coils from the surrounding atmosphere.
These conventional degreaser designs present a number of limitations. First, the condensing coils are rarely sufficient to completely prevent the escape of solvent from the device; because some of the traditional degreasing solvents (particularly chlorofluorocarbons) are now known to be environmentally hazardous or toxic to workers, the range of usable solvents has been considerably restricted in recent years. Furthermore, even accepting the inevitability of environmental release of solvent vapor, the rate of condensation must be operationally practical; excessive solvent loss can raise the cost of cleaning considerably. Therefore, refrigeration or a source of cooling water must often be provided to achieve sufficient condensation, and such features both increase the cost of operating the device and limit the amount of solvent that can feasibly be recycled.
The energy consumption necessary for cooling is increased further by the tendency of coils to condense atmospheric moisture into films on their exterior surfaces. This condensate acts as an additional layer of insulation, interfering with the efficient flow of heat and therefore increasing the energy input necessary to achieve a given degree of cooling. The problem affects not only vapor degreasers, but any type of condensing apparatus exposed to the atmosphere. This is a particular drawback in distillation apparatus, which are often used in conjunction with vapor degreasers, due to the intimate contact between the coils and the vapor that is usually necessary for effective condensation after separation.
The risk of combustion is another factor associated with many otherwise efficacious solvents that limits their usefulness with traditional vapor degreasers. Without complicated level-control sensors and associated valves, the amount of solvent within the boiling sump can decline to the point of exposing the heater, thus permitting direct contact between the heater and easily ignited solvent vapor. The danger is increased by the need for conventional degreasers to remain open to the atmosphere, permitting the entry of oxygen into the solvent-vapor zone that increases the susceptibility of the vapor to ignition.